The subject invention relates to frequency compensation of high frequency amplifiers, and particularly amplifiers having three poles.
Many types of present day solid-state, high frequency amplifiers require frequency compensation techniques. For instance, even a simple, one stage common emitter amplifier having one transistor with a current source connected to the base thereof and a further current supply connected to the collector thereof can require high frequency compensation. The high impedance of the current source driver and stray capacitance creates a first pole at the base electrode of the transistor. Moreover, a second pole is created at the collector of the transistor due to parasitic capacitance, for instance. Because the two uncompensated poles are relatively close together in the frequency domain, the gain of the amplifier rolls off at 40 dB per decade near the unity gain frequency of the amplifier. Moreover, the amplifier provides significant voltage gain when the phase shift between the input and output signals is 360.degree.. Thus, if the output terminal of the amplifier is electrically coupled to the input terminal thereof the conditions for undesirable oscillation are met.
To avoid such oscillation or instability, frequency compensating capacitors to ground have been connected to the input or output terminals of amplifiers. Also Miller or pole splitting capacitors have been utilized within such amplifiers.
Such high frequency compensation techniques have been found to be effective in and are utilized in many types of commercial monolithic integrated amplifier circuits operating at relatively low frequencies. Unfortunately, as the frequencies of such amplifiers are increased additional poles are created therein. If these new poles are spaced relatively close together or near other poles in the frequency domain, then undesirable peaking occurs in the gain frequency plot. This peaking can cause instability or oscillation. Additional poles are generated for example by the internal capacitance of transistors when the frequency of operation approaches f.sub.T, which is the frequency where the absolute value of beta equals one. In the past, this problem has been partially solved by running more current through the transistor to increase the value of f.sub.T which results in moving the third pole further out along the frequency axis. However, as the unity gain frequency of the amplifier approaches about 5 Megahertz, it no longer is practicable to further increase the frequency response by increasing the current of the amplifier transistors.